I have a large patch of asclepias incarnata that has some leaves that are turning black and some that are shriveling. Whole plants are dying off.
I showed Dr. Matt Frye, an entomologist at Cornell University, a picture of this problem. His diagnosis, a one two push of the aphids. The aphids secrete honeydew, and that caused sooty mold to proliferate. The solution, organic horticultural soap to kill the aphids. It did not hit my asclepias Syriaca so far was not affected, which is good for the Monarchs in our garden.